fix fs-misstrust on android

this is a followup to !652 (merged)

Arti would fail to get loaded on Android, because getgrnam_r (and possibly other functions) is not provided by Bionic.
Then it would fail to run in the same way as on iOS due to a group-writable directory somewhere on the path to its datadir.

I've added some references on why not verifying group-writability of the path is safe in my opinion, both for iOS and Android.